Recently, the development of new organic luminescent materials in the solid or aggregate state has attracted more attention due to their wide applications in the fields of electronics (Adv. Mater., 2011, 23, 926-952; Chem. Sci., 2011, 2, 2402; Chem. Rev. 2007, 107, 1011), optics (Adv. Mater., 2012, 24, 1703-1708), storage mediums (Adv. Mater., 2012, 24, 1255-1261), and biological sciences (Chem. Sci., 2012, 3, 984).
However, aggregation-caused quenching (ACQ) is a common problem for traditional luminescent dyes when their molecules are aggregated due to energy transfer and the formation of excimers and exciplexes. To mitigate the ACQ effect, various chemical (Chem. Commun., 2008, 1501. Chem. Commun., 2008, 217), physical, and engineering (Langmuir, 2006, 22, 4799. Macromolecules 2003, 36, 5285) approaches and processes have been developed. These attempts, however, have only resulted in limited success. The difficulty lies in the fact that aggregate formation is an intrinsic process when luminogenic molecules are located in close vicinity in the condensed phase. Accordingly, there is a great need in the art for a system where light emission is enhanced, rather than quenched, by aggregation.
In 2001, the present inventors developed such a system, in which luminogen aggregation played a constructive, instead of a destructive, role in the light emitting process. The inventors also observed a novel phenomenon and coined the term “aggregation-induced emission” (AIE) since the non-luminescent molecules were induced to emit by aggregate formation. For example, a series of propeller-like, non-emissive molecules, such as silole and tetraphenylethene (TPE), were induced to emit intensely by aggregate formation (Chem. Commun. 2001, 1740; J. Mater. Chem. 2001, 11, 2974; Chem. Commun. 2009, 4332; Appl. Phys. Lett. 2007, 91, 011111.). After this discovery, the present inventors discovered a large number of molecules bearing this novel property. In addition, through a series of designed experiments, and theoretical calculations, the present inventors identified restriction of intramolecular rotation (IMR) as the main cause for the AIE effect (J. Phys. Chem. B 2005, 109, 10061; J. Am. Chem. Soc. 2005, 127, 6335).
Since then, various kinds of AIE dye have been widely developed and applied in many fields: OLEDs (J. Mater. Chem., 2011, 21, 7210-7216; J. Mater. Chem., 2012, 22, 11018-11021), bio-probes (J. Am. Chem. Soc., 2012, 134, 9569-9572), chemosensors (J. Am. Chem. Soc., 2010, 132, 13951-13953; J. Am. Chem. Soc., 2011, 133, 18775-18784), and cell imaging (Adv. Mater., 2011, 23, 3298-3202).
However, most AIE dyes prepared so far emit blue or green light determined by their nature of structure (Chem. Commun., 2012, 48, 416; Chem. Commun., 2012, 48, 7880; Chem. Sci., 2012; J. Mater. Chem., 2012, 22, 12001), which limits the application of AIE dye, especially in the field of bioscience. The development of a new AIE dye emitting at a long wavelength is especially needed because it may tolerate little interference between optical self-absorption and autofluorescence from the background (Chem. Commun., 2012, 48, 6073-6084). As is known in the art, to achieve long wavelength emission, the dye molecules are generally constructed from merged planar rings with extended conjugation or that possess strong dipoles coming from electron-donating and accepting groups (ICT process) (Org. Lett., 2008, 10, 4175-4178). However, extending conjugation may be difficult from a synthesis standpoint. Moreover, the emission stemming from the ICT process is always weaker for traditional luminescent dyes in aqueous media due to the effect of polarity for ICT emission (Chem. Rev., 2003, 103, 3899-4032; J. Phys. Chem. C, 2009, 113, 15845-15853). This is unfavorable in the bio-environment.
Accordingly, there is a great need for the development of AIE luminogens that can emit long wavelength fluorescence.